i2c: cros-ec-tunnel: Fix slave device enumeration

During adding of the adapter the slave device registration
use to fail as the acpi companion field was not populated.

Fixes: 9af1563a

 ("i2c: cros-ec-tunnel: Make the device acpi compatible")
Signed-off-by: Akshu Agrawal <akshu.agrawal@amd.com>
Acked-by: Raul E Rangel <rrangel@chromium.org>
Reviewed-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>